This invention is concerned generally with the measurement of scattered light and more particularly is concerned with the measurement of the energy and direction of light scattered by particles passing through an optical sensing zone whereby to enable the identification of the particles and/or their characteristics.
The invention herein has relatively wide application but particularly is of value in the identification of white blood cells, cancer cells and other biological particles.
There is a considerable body of literature and prior art on the work which has been done by others in the identification of biological cells and it would be of some value to review the same briefly. It would be advantageous also to delineate the relationship of this invention with the apparatus which has been utilized and is described in the prior art.
Basically, a sensing zone is established in some way by directing a beam of concentrated light to a small volume through which the particles are to be passed, the particles are directed to pass through the zone and the scattered light is detected in different geometric locations around the zone. Scattering may occur backward or forward of the zone relative to the light source. The zone is usually called a scattering point.
In its simplest form, a stream of liquid or air carrying the particles is flowed through a pipe and at a transparent location along the pipe a beam of light is projected across the stream. A photodetector on the side of the pipe opposite the source of the beam of light will detect a change in its response each time that a particle passes. Obviously the fact of change enables the particles to be counted. The "shadow" thrown by the particle on the photodetecting device provides some information as to size. Other photodetecting devices can be positioned at locations spaced from the axis of the light beam to give signals which are related to the amount of light scatter in different polar locations. The direct beam can be blocked out and only the scattered light measured, if desired.
In biological cells, the condition of the interior of the cell will produce scattering of light in different ways and many of the apparatuses of the prior art are concerned with the method and techniques whereby the effects of light scattering help identify the cells.
Identification of the cells, especially white blood cells, is needed for diagnosis and detection of disease, for the ascertaining of patient condition and the effects of therapy, etc. Present methods and apparatus for this purpose are channeled toward the automation of the identification techniques to enable high speed measurements and positive identification. This is to enable the elimination of the slow, tedious and inaccurate manual methods that have been classically practiced in laboratories, clinics and hospitals.
The systems and apparatus which are known utilize a fluid flow which tends to pass the particles to be measured through a sensing zone one by one. Although the fluid may be a gas, generally in the study of biological particles this is a liquid such as a saline solution whose purpose importantly is to preserve the integrity and the condition of the particles. Gas and air as fluids for transporting particles to and through sensing zones are used more commonly in the study of industrial particles such as fly ash, dust, comminuted minerals etc.
Considering principally biological particles (although the prior art to be mentioned is not necessarily limited thereto) typically such particles are entrained in a sheath of liquid which is either circular or almost flat planar in cross section at the sensing zone. Several U.S. patents which disclose this type of entrainment and sensing zone are: Nos. Re. 29,141; 3,413,464; 3,657,537; 3,705,771; 3,785,735 and 3,791,196.
After the particle passes into the sensing zone, the light or other radiant energy which has been directed at the sensing zone by some means such as a concentrated lamp beam or a laser is measured at different locations relative to the sensing zone. Typical of these devices are several of those mentioned above as well as in U.S. Pat. No. 3,835,315. A system for such measurements is disclosed in U.S. Pat. No. 4,070,113 although the photodetector therein is not described in much detail.
The problem of measuring the scattered light at different locations has been attacked by others but three important disadvantages have been difficult to overcome. The first is the disadvantage of not being able to get enough information because of the difficulty of measuring a plurality of points. The second is the disadvantage of complex and difficult to manufacture apparatus with its attendant companion disadvantage of great expense. The third is the disadvantage of not getting enough energy from the scattered light at all measuring points to give meaningful data.
Each of the four prior art references mentioned hereinafter has one or more of these disadvantages.
The oldest of these references is U.K. Pat. No. 137,637 of 1920 to Pollard which utilizes expensive conical frustums and reflecting prisms. The scattered light is viewed by a microscope and/or measured by crude means compared to those available at the present time.
The second of these references is Frommer U.S. Pat. No. 3,248,551 which utilizes a compound type of annular reflector that has two surfaces and concentrates the scattered light captured by the respective surfaces and reflects same to separate photomultiplier tubes. It is quite obvious from an examination of this patent that the twosurface reflecting device is most difficult and complicated to manufacture; hence one which would require collection from many more than just two angles or polar regions would be even more difficult and expensive to manufacture. In this structure, the collection and deviation of the scattered radiant energy is effected by a single element.
Neither the Pollard nor the Frommer patent has the simplicity and efficiency of the present invention. The number of regions of light scatter from which information can be obtained is severely limited in these prior art devices.
The third and fourth of these references comprise two publications describing a device which is mentioned in U.S. Pat. No. 4,070,113 as a type of photovoltaic detector which has concentric rings formed on a disc that is several inches in diameter. The light from the scattering zone is permitted to fall directly onto this detector which then provides electrical signals related to the energy of the light at different distances from the center of the beam. The publications are an article entitled "Light-Scattering Patterns of Isolated Oligodendroglia" By R. A. Meyer, et al in The Journal of Histochemistry and Cytochemistry, Vol. 22, No. 7, pp 594-597, 1974 and a second article entitled "Gynecologic Specimen Analysis by Multiangle Light Scattering in a Flow System" by G. C. Salzman et al in the same Journal, Vol. 24, No. 1, pp 308-314, 1976. In the articles reference is made to the same or a similar detector device which is identified as a Recognition Systems, Inc. detector.
The ring detector which has been described above is quite expensive at the present time. It typically comprises 64 photodiodes arranged in rings and wedges, all on the same substrate. If any element or increment of the detector fails or is damaged the entire device may have to be discarded. Additionally, the contacts for the diodes are brought out to a narrow edge segment at which point they are required to be connected into electrical circuitry. This is a delicate and precise operation not easily effected by unskilled technicians.
Additionally, the inner rings are very small while the outer rings are quite large. Thus the radiant energy is weakly diffused over the outer rings giving low power density. Additionally the electrical capacitance of the outer rings is substantially high which results in loading and deterioration of signal. This is a problem where the particles which move through the sensing zone at high speed generate light pulses which may be as short as a microsecond.
The basic difference between the invention and the methods and apparatus which are known lies in the manner in which control of the scattered light is achieved.
The invention herein solves the problems of the prior art to eliminate the disadvantages thereof through the use of a composite spherical mirror which receives the scattered radiant energy from a sensing zone and deviates specific geometric areas thereof to different locations, the locations being spaced from one another and the deviation being in a direction folded back towards the sensing zone. The fact that there are different locations where the radiant energy is caused to confluence enables the use of totally independent photoconductive devices at the respective locations for measurement of the specific portions of radiant energy from the different geometric areas.
The independent photoconductive devices are located in any convenient array, are conventional in construction and hence are highly economical and easily replaced independently. The capacitance to ground is low permitting rapid voltage change and good response thereby preserving the amplitude of electrical signals resulting from the high speed passage of particles. The light is concentrated on a small photosensitive area resulting in the highest power density possible with consequently high signal-to-noise ratio.
The invention permits of considerable latitude in configuration, placement, construction and arrangement thereby providing high flexibility for almost any kind of system, but also with no loss in convenience and economy of use.